Transmission x-ray scanner designs for whole-body radiographic imaging have been described in the patent literature for over fifty years. One such system is described in U.S. Pat. No. 3,101,407 to Shipman, which issued on Aug. 20, 1963. In Shipman, the source and the camera are moved directly with cables and pulleys connected to a motor. Scanning starts from a parked stop, accelerates to a scanning speed, and then decelerates to a parked stop. This change in motion is repeated for the next scan. This movement introduces error into the detected image and produces motion artifacts in the image. The scanner taught by Shipman suffers from several limitations. One of the primary limitations is the complexity of the positioner (drive and cable mechanisms) used to move the x-ray source and linear x-ray camera and support the person being scanned. Such a drive-and-cable system is too complex to be practical or cost effective. The system also was designed to move slowly. Another limitation is that the motion of the x-ray source and linear x-ray camera can only move in one direction at a time, either up or down. Once the source/camera arrives at a limit of travel in either direction, it must come to a full stop and reverse direction. This creates a need to accelerate the x-ray source and linear x-ray camera when starting and stopping the scan, which also introduces the problem of gear backlash that must be compensated for when reversing the motor direction.
Another prior art system is described in U.S. Pat. No. 8,477,902 to Li et al., issued on Jul. 2, 2013. This patent describes a system having two independent vertical towers: one tower for the x-ray source and a second tower for a linear x-ray camera. The x-ray source and the linear x-ray camera are each driven by separate motors and are kept in alignment by a phasemeter. This technology has limitations with respect to vertical travel and maintaining alignment. More specifically, the motors require a start and stop distance that has effect on the scanning travel distance that can be accomplished at a constant speed. In this design, motor backlash is an issue and must be addressed and compensated for when reversing direction.